2 * util/rtt.c - UDP round trip time estimator for resend timeouts.
4 * Copyright (c) 2007, NLnet Labs. All rights reserved.
6 * This software is open source.
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39 * This file contains a data type and functions to help estimate good
40 * round trip times for UDP resend timeout values.
45 /* overwritten by config: infra_cache_min_rtt: */
46 int RTT_MIN_TIMEOUT = 50;
47 /** calculate RTO from rtt information */
49 calc_rto(const struct rtt_info* rtt)
51 /* From Stevens, Unix Network Programming, Vol1, 3rd ed., p.598 */
52 int rto = rtt->srtt + 4*rtt->rttvar;
53 if(rto < RTT_MIN_TIMEOUT)
54 rto = RTT_MIN_TIMEOUT;
55 if(rto > RTT_MAX_TIMEOUT)
56 rto = RTT_MAX_TIMEOUT;
61 rtt_init(struct rtt_info* rtt)
65 rtt->rto = calc_rto(rtt);
66 /* default value from the book is 0 + 4*0.75 = 3 seconds */
67 /* first RTO is 0 + 4*0.094 = 0.376 seconds */
71 rtt_timeout(const struct rtt_info* rtt)
77 rtt_unclamped(const struct rtt_info* rtt)
79 if(calc_rto(rtt) != rtt->rto) {
80 /* timeout fallback has happened */
83 /* return unclamped value */
84 return rtt->srtt + 4*rtt->rttvar;
88 rtt_update(struct rtt_info* rtt, int ms)
90 int delta = ms - rtt->srtt;
91 rtt->srtt += delta / 8; /* g = 1/8 */
93 delta = -delta; /* |delta| */
94 rtt->rttvar += (delta - rtt->rttvar) / 4; /* h = 1/4 */
95 rtt->rto = calc_rto(rtt);
99 rtt_lost(struct rtt_info* rtt, int orig)
101 /* exponential backoff */
103 /* if a query succeeded and put down the rto meanwhile, ignore this */
107 /* the original rto is doubled, not the current one to make sure
108 * that the values in the cache are not increased by lots of
109 * queries simultaneously as they time out at the same time */
111 if(rtt->rto <= orig) {
113 if(rtt->rto > RTT_MAX_TIMEOUT)
114 rtt->rto = RTT_MAX_TIMEOUT;
118 int rtt_notimeout(const struct rtt_info* rtt)
120 return calc_rto(rtt);